The circuit designs of the latest electronic equipment, such as TV sets, video equipment, computers, medical appliances, office equipment and communications devices, are becoming increasingly complicated, with integrated circuits containing the equivalent of several hundreds of thousands of transistors now being produced. The trend in electronic equipment toward smaller sizes and higher functionality is accompanied by an increase in the number of electronic components incorporated into the ever-shrinking footprint. At the same time, miniaturization continues also in the shapes of the electronic components themselves. As a result, the heat generated by each electronic component increases. Such heat can lead to failure or malfunction; hence the importance of packaging technology that effectively dissipates heat.
As the level of integration has increased in CPUs, driver ICs, memories and other electronic components used in electronic equipment such as personal computers, DVD players and cell phones, numerous methods for dissipating heat and heat-dissipating members adapted for use therewith have been described in order to remove the heat that is generated.
One approach hitherto taken to hold down the temperature rise by electronic components in electronic equipment has been to conduct heat directly to a heat sink using a metal having a high thermal conductivity such as aluminum, copper or brass. This heat sink carries away heat generated by an electronic component and discharges the heat from a surface by utilizing the temperature difference with outside air. To efficiently conduct heat generated by an electronic component to a heat sink, it is necessary for the heat sink and the electronic component to be placed in close contact without an intervening gap. To this end, a low-hardness heat-conductive sheet having flexibility or a heat-conductive grease is placed between the electronic component and the heat sink.
Yet, although a low-hardness heat-conductive sheet has an excellent handleability, achieving a small thickness is difficult. In addition, because it is unable to conform to minute irregularities in the surfaces of the electronic component and heat sink, the contact thermal resistance is large, as a result of which such a sheet cannot efficiently conduct heat.
By contrast, with heat-conductive greases, the ability to achieve a small thickness makes it possible to reduce the distance between the electronic component and the heat sink. Moreover, such greases bury minute surface irregularities, enabling the thermal resistance to be greatly reduced. However, a drawback of heat-conductive greases is that, with thermal cycling, the thermal properties of the grease decline due to separation of the oil component (pump-out).
In recent years, many thermosoftening materials that are solid at room temperature and soften or melt due to heat generated by electronic components have been described as heat-conductive members which are endowed with both the good handleability of a low-hardness heat-conductive sheet and the thermal resistance-lowering effect of a heat-conductive grease, and additionally provide the advantage of a better pump-out resistance than conventional thermal greases (Patent Documents 1 to 7: JP-A 2000-509209, JP-A 2000-336279, JP-A 2001-89756, JP-A 2002-121332, JP-A 2000-327917, JP-A 2001-291807, JP-A 2002-234952).
However, because thermosoftening materials in which the base oil is based on an organic substance (Patent Documents 1 to 4) have an inferior heat resistance, when such heat-conductive members are incorporated into automotive applications, for example, there is a concern over deterioration at elevated temperatures. Many similar thermosoftening materials that are based on silicones have also been described as materials endowed with good heat resistance, weatherability and flame retardance (Patent Documents 5 to 7), but these are all sheet-type materials.
In cases where the heat-dissipating material is rapidly arranged over a large surface area, air pockets inevitably form between a sheet-type material and the surface with which it comes into contact. Moreover, from the standpoint of the work involved during such arrangement of the heat-dissipating material, the most rapid and efficient method is to apply a heat-dissipating material in paste form by a technique such as screen printing. Sheet-type materials leave something to be desired from this standpoint as well.